Included files, all or nothing?

Question

I have been writing code for a while, but I am not classically trained in computer science, so if this question is ridiculous, please go easy on me.

Something I have been trying to find a definitive answer on for a while is, if I #include a file in C, do I get the ENTIRE contents of the file linked in, or just the parts I use? If it has 10 functions in it, and I only use 1 of the functions, does the code for the other 9 functions get included in my executable? This is especially relevant for me right now as I am working on a micro-controller and memory is precious.

Thanks for any help with this question.

Answer 1

Firstly, header files do not get "linked in". #include is basically a textual copy-paste feature. Everything from your include file gets pasted by preprocessor into the final translation unit, which will later be seamlessly processed by the compiler proper. The compiler proper knows nothing about any header files or #include directives.

Secondly, it means that if in your code you declared or defined some function or variable that you do not use, it is completely irrelevant whether it came from a header file through #include or was written directly in source file. There's absolutely no difference.

Thirdly, the question is: what exactly do you have in your header file that you include? Typically, header files do not define objects and functions, they simply declare them. Declarations do not produce any code, regardless whether you use the function or not. Declarations simply tell the compiler that the code (generated from the function definition) already exists elsewhere. Thus, as long as we are talking about typical header files, #include directives and header files by themselves have no effect on final code size.

Fourthly, if your header file is of some unusual kind that contains function (or object) definitions, then see "firstly" and "secondly" above. The compiler proper can see only one translation unit at a time, for which reason a typical strategy for the compiler proper is to completely discard unused entities with internal linkage (i.e. static objects and functions) and keep all entities with external linkage. Entities with external linkage cannot be discarded by compiler proper, since they might be needed in some other translation unit.

Fifthly, at linking stage linker can see the program in its entirety and, for that reason, can discard unused objects and functions, if it is advanced enough for that (and if you allow linker to do it). Meanwhile, inclusion-exclusion precision of a typical run-of-the-mill linker is limited to a single object file. Each object file is atomic to such linker. This means that if you want to be able to exclude unused functions on per-function basis, you might have to adopt "one function per object file" strategy, i.e. write one and only one function per .c file. Of course, this is only possible when you write your own code. If some third-party library you want to use does not adhere to this convention, then you might not be able to exclude individual functions.

Answer 2

If you #include a file in C, the entire contents of that file are added to your source file and compiled by your compiler. A header file, though, usually only has declarations of functions and no definitions (so no actual code is compiled).

The linker, on the other hand, takes all the functions from all the libraries and compiled source code and merges them into the final output file. At this time, the linker will discard any functions that you aren't using.

So, to answer your question: only the functions you use (and indirectly depend on) will be included in your final program file, and this is independent of what files you #include. Happy hacking!